The total costs of musculoskeletal disease in the US in 2000 have been estimated at US $254 billion and, in developing countries, the figure is estimated at US $100 billion. Under normal conditions, muscles can repair themselves by regenerating damaged muscle fibers and restoring muscle strength. Following an initial necrosis of damaged muscle fibers, an inflammatory response is initiated that activates a resident population of quiescent cells termed satellite cells. These myogenic cells proliferate, migrate to the site of injury, differentiate and fuse to form mature myofibers, or fuse with existing myofibers thus regenerating damaged muscle fibers and restoring their function. When these normal processes are compromised by disease or age, damaged muscle fibers are instead replaced by infiltrating fibrous tissue or fat, leading to a net loss of muscle mass and a resultant loss of strength.
Cell transplantation has been used in regenerative medicine for musculoskeletal disorders as well as degenerative conditions such as diabetes with limited success. Limitations of earlier approaches include loss of cell viability and function following transplantation.